1. Field of the Invention
This invention concerns thick film print head apparatus and electronic control therefor and in particular to improvements for thermal recording on thick film. Still more particularly, the invention concerns a novel printhead structure and novel driver ICs (Integrated Circuit) with incorporation of a variety of control methods for recording and energy control.
2. Description of the Prior Art
Prior Printhead Structures
On a thermal printhead the heater nib line is made of an array of heater elements, on which Joule's heat is generated by applying a voltage across and flowing the current through the heater element. The traditional structure (Durbeck et al., “Output Hardcopy Devices,” Academic Press, 1988) is shown in FIG. 1 where conductive paths are connected to both ends of each heater element, 0101a, . . . , 0101e, also denoted as resistive elements R1, . . . , R5, with the supply power supply Vhd 0103. The resistive elements are called “nibs”. A third conductive path is connected from the center of each heater element to a switch, 0102a, . . . , 0102e, usually embedded in a driver IC (Integrated Circuit) which is connected to ground.
The switch is turned on (0102b, 0102e) or off (0102a, 0102c, 0102d), depending on the corresponding nib's data bit being 1 or 0. The operation is quite straightforward—when the switch is on, electrical currents are flowing from both ends of a heater element towards the center of the heater element, and thereby Joule's heat is generated on the heated nibs 0101b, 0101e. Hence this structure is referred to as a “center-tap” arrangement. There are two drawbacks to this arrangement, however. First, the density of the conductive pattern is twice the resolution of the printed dots. Second, each printed dot may be of an undesirable butterfly-like shape. This is because usually high electrically conductive material such as gold is used as the electrodes on a thick film thermal head and, due to high thermal conductivity of gold, heat escapes from the conductive pattern in the center portion toward the electrodes, resulting in lower temperature than other parts of the heater element.
One way to reduce the density of the conductive pattern and also to eliminate the undesirable butterfly-like dot shape is to provide an alternated conductive lead system. A simplified circuit diagram of such a system (Tanno et al., U.S. Pat. No. 3,984,844, 10/1976; Mizuguchi et al., U.S. Pat. No. 4,141,018, 2/1979) is shown in FIG. 2 where the nominal resistance of each heater element (i.e., nib) is denoted as R. The addition of diodes 0204 in the upstream conductive path 0203 prevents the leakage current from reverse-flowing. Each nib of the heater nib line 0205 is turned on or off, by a switching circuit 0206 embedded in a driver IC, depending on the raster data bit being 1 or 0 associated with a specific nib. The downstream switches 0206 and the upstream conductive circuits 0203 are shared by pairs of neighboring nibs. When the external controller sends raster data to the register in the driver IC, the power supply 0207 is connected to the conductive path A 0201, termed “A-phase,” or path B 0202, termed “B-phase,” alternatively. In this kind of configuration the density of conductive pattern is the same as the resolution of printed dots. However, there exists undesirable leakage current. For example, while the current flowing through the supposedly energized nib 0208 is V/R as designed, the leakage current flowing through the three neighboring nibs 0209, 0210, 0211 on the left side is V/(3R), thereby generating 1/9 of Joule's heat as of that on the energized nib 0208.
For high-resolution thermal heads, the space allowed for mounting diode arrays becomes constrained. Consequently, a deep diffusion zone is required during the semiconductor manufacturing process. Therefore, adding diode arrays as in FIG. 2 presents problems in technical implementation as well as an increase in manufacturing cost. Another thermal printhead development has focused on removing the diodes and adding a secondary power supply (Kos, U.S. Pat. No. 4,032,925, 6/1977; Yeung, U.S. Pat. No. 5,134,425, 7/1992; Watanabe et al., U.S. Pat. No. 5,702,188, 12/1997; Smither et al., U.S. Pat. No. 5,805,195, 9/1998). FIG. 3 shows such an arrangement for a diode-less alternated conductive lead system, where VP is the voltage from a primary power supply and VS (<VP) is applied from a secondary power supply. In the example, a single switch 0303 is turned on for heating nib R8, and switches 0301 and 0302 are turned on for heating nibs R3 and R4.
If all the nibs are of the same resistance value (nominal R), IE=VP/R is the current designed to flow through each nib to be heated (R3, R4 and R8), IN=VS/R is the current through the neighbor nibs (R2, R5 and R9), and IO=(VP−VS)/(2R) is the current through all the other nibs (R1, R6, R7, R10, R11, R12, . . . ), as described in a published paper (Toyosawa et al., “Development of dual-line wide-format 1200-dpi thermal printhead,” Journal of Imaging Science and Technology (JIST), vol. 53, no. 9, September/October 2009). If VS is selected to be of ⅓ of VP, as shown in FIG. 4, all the nibs which are not to be energized intentionally would have the same parasitic current as ⅓ of the full current and an undesirable Joule's heat of 1/9 of the full-power on the energized nibs (R3, R4 and R8), no matter what data pattern is applied on the driver ICs.
In most thermal printing applications, on paper or film, 1/9 of full power is below the threshold of the energy curve for activating a heater element (nib) to mark a visible dot on the media, thus no spurious printing would result. However, the undesirable residual heat of 1/9 full power may pose a problem for certain applications. For example, in DTS (Direct-To-Screen) thermal systems, images are to be printed on a screen by transferring heat from the thermal head to a ribbon and then to the top emulsion of the screen. A successful complete DTS job requires a clean and sharp peel-off of the unheated ribbon from the screen, but the residual heat generated by 1/9 of full power may raise the temperature in the unprinted ribbon area, resulting in a sticky image edge and an unclean peel-off.
The issue of the undesirable residual heat generated by the leakage current can be examined by looking at the voltage difference across a single nib heater element 0501, as shown in FIG. 5 where the upstream conductive lead 0502 is usually connected to a power supply (i.e., VU=Vhd) and the downstream conductive lead 0503 to a driver IC. In the desirable condition, when the data bit corresponding to the nib is 1, the switch on the driver IC is enabled and the downstream lead is connected to ground so that VD=0.
However, when the data bit is 0, the switch on the driver IC is disabled (i.e., open) and the downstream lead “floats”. The values of VU and VD vary, depending on the printhead circuitry structure and the location of the supposedly unheated nib. For example, in alternated conductive lead system with diodes, as shown in FIG. 2, when the switch for a downstream lead is open, there are three possible cases: a) VU=VD=V, b) VU=V, VD=⅔V, or c) VU=⅓V, VD=⅔V. In the two latter cases a leakage current and undesirable heat are generated. Similarly, in a diode-less alternated conductive lead system (FIG. 3) assuming VS=VP/3, when the switch for a downstream lead is open, it could be a) VU=VP, VD=(VP−VS)/2=VP/3, or b) VU=VS, VD=(VP−VS)/2=VP/3. In both cases a leakage current and undesirable heat are generated.
Prior Driver ICs
The prior art block diagram of FIG. 6 is for a typical off-the-shelf 64-bit driver IC 0601 comprising a shift register 0602, latches 0603 and gated switches 0604. IC 0601 reads the raster data into the SerialIn port, shifts the data bit by bit, and after all the data bits have been shifted in, enables the latch and strobe signals so that all the output switches 0604 are turned on or off according to the raster data pattern. The output ports DO1 . . . DO64 are connected to the downstream conductive lead 0503 (see FIG. 5) of each nib on the heater nib line. Therefore for the nib bit designated as 1, the gated switches of FIGS. 2 and 4 are on, the downstream conductive lead is connected to ground, the current flows through the corresponding nib and Joule's heat is generated. The timing diagram of control and data signals for the diode-less alternated conductive lead system is shown in FIG. 7 along with the switching timing of the power supplies VP and VS (=VP/3) for A-phase and B-phase.
For real system implementation, a driver IC can accommodate only a limited number of parallel outputs and switches. Hence multiple driver ICs are used. Usually a full scanline of raster data are fed into a raster data processing FPGA (Field Programming Gate Array) which splits a full scanline into multiple bit streams of short length equal to the data width of the driver IC. The FPGA then sends these in-parallel bit streams simultaneously along with a set of control signals (Clock, Latch, Strobe, etc.) to the driver ICs. FIG. 8 is a block diagram 0801 for such a prior art arrangement utilizing multiple 256-bit driver ICs 0804a, 0804b of which the structure is similar to the 64-bit driver IC 0601 of FIG. 6. One advantage of the structure (FIG. 8) of multiple in-parallel bit streams is the increase of printing speed, because the processing time is only limited by the data width of driver ICs. For example, when 256-bit driver ICs are used, it takes only 256 clocks to shift a full scanline of raster data and load into the heater nib line 0802. During the time period of data shifting and loading, the raster data processing FPGA 0803 may also simultaneously accomplish tasks of data sorting and bit stream splitting for the next scanline.
In the alternated conductive system, either with diodes (FIG. 2) or without diodes (FIG. 4), conductive leads to the power supply are usually connected to the opposite side to those connected to the driver ICs. This symmetric structure of conductive leads is more suitable for flat-type printheads, but may make manufacturing of edge or near-edge type thermal heads more inconvenient.
Prior Printhead Energy Control Methods
An imperative requirement for thermal printing is to have a consistent and uniform dot shape across the whole printed image. One affecting factor is the temperature of the printhead because the thermal head gets warmed up and the dot size grows bigger as the printing continues, due to the nature of heat dissipation and accumulation on the nib line. To ensure that the heated nibs have the same dot size during the cold-start period (i.e., beginning of printing) as in the steady-state period (i.e., after getting warmed up and the thermal head stays at a constant temperature), two approaches of energy adjustment based on thermistor readings have been used. One method is to adjust the voltage level of the power supply which provides the current for heating the nibs. Usually it is done by the main processor which adjusts the power supply through remote control signals. This is shown in FIG. 9 which is a block diagram of a printer system utilizing a thermal head, with the motion mechanism and corresponding motion controller omitted for clarity of illustration. The other approach is to apply additional small pulses to the nibs, thus generating more heat, during the cold-start period.
The most important factors affecting the consistence and uniformity of dot shape are the residual heat of the nib due to heating in the previous data line as well as the heat transfer from the neighbor nibs due to leakage current. To compensate for such effects (which are the characteristics of the thermal head), a multi-pulse control strategy, commonly known as history control or hysteresis control, is usually adopted. It appends a number of small pulses, following the main pulse (corresponding to nib data's being 1), to the driver IC so that additional current is applied to the nib. The control pattern, i.e., the number of those small pulses and the pulse width, depends on the historical data (i.e., nib data of previous lines) as well as the data pattern of neighbor nibs.
FIG. 10A is an example timing diagram illustrating that each nib receives one main voltage pulse 1001 corresponding to the raster data bit being 1 or 0, three small pulses 1002, 1003, 1004 for hysteresis control and two small pulses 1005, 1006 for temperature compensation. The implementation of multi-pulse control usually resides in the raster data processing FPGA which sends the data bit streams of a full scanline one by one for the pulse train. FIG. 10B illustrates that the sequence of sending the bit streams 1012, . . . , 1016 for pulses used in hysteresis control and temperature compensation are essentially the same as the bit stream 1011 for the main pulses.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a new structure for a thick film printhead.
Another object of the invention is to provide a new driver IC as well as a variety of control methods for the new thick-film printhead identified above
Another object is to provide an assembly wiring structure so that all conductive leads are on the same side of the nib line in the new printhead identified above.